FIG. 13 is a flow chart showing the focus algorithm of a stationary exposure apparatus (stepper). A focus on a wafer as a substrate loaded onto a wafer stage is measured at a focus measurement shot out of several representative shots on the wafer (step 1301). A global approximate plane as a linear plane which represents the wafer surface is calculated by, e.g., least-square approximation from the obtained focus measurement data (step 1302). The target value of the wafer stage in the Z direction when the wafer stage is driven to locate the wafer surface on the global approximate plane is called a global focus target value, and its target values in the ωx and ωy directions at this time are called global tilt target values.
Shots subjected to global focus/tilt measurement simultaneously undergo alignment measurement at, e.g., alignment mark positions (1401x, 1401y, 1402x, 1402y, 1403x, 1403y, 1404x, and 1404y) of shots hatched on the wafer in FIG. 14. FIG. 14 is a view showing a shot position on the wafer subjected to global focus measurement. To increase the precision of the measured global tilt amount and that of the rotation angle in the θ direction in global alignment, measurement spans in the X and Y directions are preferably widened, and the shots are desirably near the periphery of the wafer.
In FIG. 13, an X-Y stage moves stepwise to a target exposure shot position (step 1303). At the same time, a Z tilt wafer stage moves to a focus/tilt position in the exposure shot that is estimated from the global approximate plane (step 1304). Upon the completion of movement, a focus is measured again at the exposure position (step 1305). Focusing driving (step 1307) and focus measurement (step 1305) are repeated until the focus tolerance check passes (step 1306). In focus measurement (step 1305), a focus sensor measures focus measurement points in a plurality of exposure shots as shown in FIG. 15, and a linear approximate plane obtained from the measurement values is set as a wafer surface. Wafer stage target values for performing focusing so as to locate the wafer surface on an image plane are sequentially calculated. FIG. 15 is a view showing the layout of focus measurement points within an exposure shot in the stationary exposure apparatus.
In FIG. 15, ch1 to ch5 are focus measurement points, one measurement point is formed from five marks (1501a to 1501e), and the average of the measurement values of the respective marks in the focus direction is used as focus measurement data for each point (ch1 to ch5). An exposure shot 1502 is measured at a plurality of focus measurement points (ch1 to ch5). For example, when the focus measurement value of ch5 greatly deviates from those of ch1 to ch4, the wafer surface is regarded to have a defect and is not exposed (case A). Alternatively, as for a greatly different measurement point, its measurement value is not used as focus measurement data, and an approximate plane is calculated from the remaining focus measurement points (case B).
If the focus tolerance passes, the flow waits for convergence of residual vibrations in the X and Y directions generated in the step of focus driving/Abbe correction driving (step 1307) in FIG. 13 (step 1308), and exposure starts (step 1309). In step 1310, whether all the shots have undergone the exposure sequence is checked. If N in step 1310, the focus algorithm (steps 1303 to 1310) is repeated by using the first measured global approximate plane data (step 1302).
The surface of a process wafer may be corrugated by about 5 to 10 [μm] during the manufacturing process. When such a wafer is to be exposed, the exposure sequence is interrupted at the shot of a corrugated portion, and the shot does not allow exposure in a prior art like case A. In this case, in the prior art, the exposure sequence shifts to process the next shot without exposing the shot of a projecting portion. At the unexposed shot, the residual resist amount is larger than that at peripheral shots in the resist developing process, and may adversely affect a resist image at the peripheral shots.
If a focus error is generated by a wafer, the job aborts in the prior art, and the exposure sequence enters a manual operation mode where the operator determines processing of the defective shot.
If a focusing error is generated under the influence of dust or the like attached to a wafer chuck, the chuck must be cleaned to remove its contamination. In a prior art like case B, however, no focusing error is detected, and the contamination of the wafer chuck must be estimated from the wafer developing result.